1. Field of the Invention
The present invention relates to a light application apparatus, a crystallization apparatus and an optical modulation element assembly, and to, e.g., a technique which generates a crystallized semiconductor film by applying a laser light having a predetermined light intensity distribution to a non-single-crystal semiconductor film or layer such as a polycrystal semiconductor film or an amorphous semiconductor film.
2. Description of the Related Art
A thin film transistor (TFT) which is used for a switching element or the like which selects a display pixel in, e.g., a liquid crystal display (LCD) has been conventionally formed in an amorphous silicon layer or a polysilicon layer.
The polysilicon layer has a higher mobility of electrons or electron holes than the amorphous silicon layer. Therefore, when a transistor is formed in a polysilicon layer, a switching speed is increased and a display response speed is thus increased as compared with a case where a transistor is formed in an amorphous silicon layer. Further, a peripheral LSI can be formed of a thin film transistor. Furthermore, there is an advantage that a design margin of any other component can be reduced. Moreover, peripheral circuits such as a driver circuit or a DAC can be operated at a higher speed when these peripheral circuits are incorporated in a display.
Since polysilicon is formed of an aggregation of crystal grains, when, e.g., a TFT transistor is formed, a crystal grain boundary exists in a channel area of this transistor, and this crystal grain boundary becomes an obstacle and lowers the mobility of electrons or electron holes as compared with single-crystal silicon. Additionally, in case of many thin film transistors formed to polysilicon, the number of crystal grain boundaries formed in channel portions differs between the respective thin film transistors, this becomes unevenness and leads to a problem of display irregularities in case of a liquid crystal display. Thus, in recent years, in order to improve the mobility of electrons and electron holes and reduce irregularities in the number of crystal grain boundaries in channel portions, there has been proposed a crystallization method which generates crystallized silicon having crystal grains with a particle size which is as large as at least one channel area can be formed from non-single-crystal silicon.
As this type of crystallization method, there has been conventionally known a “phase control ELA (excimer laser annealing) method” which generates a crystallized semiconductor film by applying an excimer laser light to a phase shifter which is closely arranged in parallel to a polycrystal semiconductor film or an amorphous semiconductor film. The detail of the phase control ELA method is described in, e.g., Journal of the Surface Science Society of Japan Vol. 21, No. 5, pp. 278-287, 2000.
In the phase control ELA method, a light intensity distribution having an inverse peak pattern (a pattern in which a light intensity is minimum at the center and the light intensity is suddenly increased toward the periphery) in which the light intensity is lower than that at the periphery is generated at a point or a line corresponding to a phase shift portion of a phase shifter, and a laser light ray having this inverse-peak-shaped light intensity distribution is applied to a non-single-crystal semiconductor film (a polycrystal semiconductor film or an amorphous semiconductor film). As a result, a fusing area having a temperature gradient according to the light intensity distribution is generated in an irradiation target area, a crystal nucleus is formed at a part which is solidified first or not solidified in accordance with a point where the light intensity is minimum, and a crystal grows in a lateral direction from this crystal nucleus toward the periphery (which will be referred to as a “lateral growth” or a “growth in a lateral direction” hereinafter), thereby generating a crystal grain with a large particle size.
Further, there has been conventionally a crystallization method for a large particle size described in M. NAKATA and M. MATSUMURA, “Two-Dimensionally Position-Controlled Ultra-Large Grain Growth Based on Phase-Modulated Excimer-Laser Annealing Method”, Electrochemical Society Proceeding Volume 200-31, page 148-154. In this method, an element having a pattern which forms, e.g., a V-shaped light intensity gradient distribution and an element having a pattern which forms an inverse-peak-shaped light intensity minimum distribution are both realized by providing a phase step on an SiO2 substrate. Furthermore, an excimer laser light is applied in a state where a processed substrate is in close proximity to the two laminated elements, thereby generating a crystallized semiconductor film on the processed substrate.
Moreover, there is a crystallization method for a large particle size described in “Silicon thin film amplitude/phase-controlled excimer laser fusing/re-crystallization method—new two-dimensional position-controlled large grain formation method” by Inoue, Nakata and Matsumura, The institute of Electronics, information and Communication Engineers Transaction, The institute of Electronics, information and Communication Engineers, August 2002, Vol. J85-C, No. 8, p. 624-629. In this method, an element having a pattern which forms, e.g., a V-shaped light intensity gradient distribution is realized by a thickness distribution of SiONx which is a light absorption material, and an element having a pattern which forms an inverse-peak-shaped light intensity minimum distribution is realized by a phase step of SiO2. These two elements are laminated and formed on one substrate. Additionally, an excimer laser light is applied in a state where a processed substrate is in close proximity to this one element substrate, thereby generating a crystallized semiconductor film on the processed substrate.
In the conventional technique, when a phase shifter having a phase step of 180 degrees is used, there is a disadvantage as described below with reference to FIGS. 44A and 45.
A crystallization apparatus in which an image formation optical system is provided between a phase shifter 191 shown in FIG. 44A and a processed substrate and an image of the phase shifter 191 is formed on a predetermined surface of the processed substrate by the image formation optical system, as shown in FIG. 44B, a minimum light intensity (a light intensity at an inverse peak point) 192 in a light intensity distribution having an inverse peak shape formed on the processed substrate through the image formation optical system is dependent on a phase difference obtained by a step 193 of the phase shifter 191. As shown in FIG. 44C, when a phase shifter having a phase difference of 180 degrees obtained by a step 193 is used, a light intensity distribution with an inverse peak shape formed at a focus position (an image formation surface) of the image formation optical system is symmetrical, and its minimum light intensity is substantially zero.
Further, an inverse-peak-shaped light intensity distribution to be formed is likewise symmetrical at a defocus position slightly moved in the vertical direction from the focus position of the image formation optical system as shown in FIG. 44D, and its minimum light intensity becomes slightly stronger but it is a very small light intensity. When a phase shifter having a phase difference of 180 degrees is used in this manner, since the symmetry of the light intensity distribution is maintained without being dependent on a defocus direction, a deep focal depth can be realized. Since the minimum light intensity is very weak at the inverse peak point, however, there is a disadvantage that an irradiation target area with the minimum light intensity is not fused, an uncrystallized area (an area having a smaller light intensity than that at a crystal growth start point) becomes large to some extent and a filling rate of a crystal grain cannot be increased. That is, almost all of an irradiation target surface can be fused by selecting a minimum light intensity in such a manner that a temperature of the irradiation target area generated when irradiated with the minimum light intensity becomes a temperature in the vicinity of a fusing point, and a crystallized area can be widened.
A step which is used to form the phase shifter 191 having a desired phase difference is obtained from an expression λ/(θ/360)/(n−1), wherein λ is a wavelength of a laser light, θ is a value which represents a desired phase difference by degree, and n is a refraction factor of a transparent quartz base material of the phase shifter. When a refraction factor of the quartz base material is 1.46 and a wavelength of an XeCl excimer laser light is 308 nm, a step of 334.8 nm must be formed to the quartz substrate by a method such as etching in order to provide a phase difference of 180 degrees. When a phase shifter in which a step 193 is selected to obtain a phase difference of 60 degrees is used as shown in FIG. 45A, a light intensity distribution with an inverse peak shape formed at a focus position of the image formation optical system is symmetrical (symmetrical in the lateral direction) as shown in FIG. 45C, and its minimum light intensity is strong to some extent. On the contrary, at defocus positions slightly moved upwards and downwards from the focus position of the image formation optical system, the symmetry of the light intensity distribution with the inverse peak shape to be formed largely collapses, and a position of its minimum light intensity (an inverse peak point) moves in the lateral direction. Here, a board thickness deviation which can be a factor of defocusing unavoidably exists in the processed substrate.
As described above, the phase shifter 191 having a phase difference of 60 degrees (FIG. 45A) has a slightly stronger minimum light intensity at an inverse peak point than the phase shifter (FIG. 44A) having a phase difference of 180 degrees, and hence a crystallized area can be widened. However, the symmetry in the lateral direction greatly collapses in the light intensity distributions at defocus positions moved upwards and downwards from the focus position, and the symmetry collapsing directions in the light intensity distributions shown in FIGS. 45B and 45D are opposite in dependence on the defocus direction, and the focal depth thereby becomes shallow (narrow). Furthermore, since a position of the inverse peak point moves in a surface (an up-and-down direction and a right-and-left direction in the drawing) due to defocusing, a position of a crystal grain to be generated is also shifted from a desired position, which disadvantageously results in a problem when forming a circuit in a formed crystal grain. That is, if a crystal grain is not formed at a desired position, a channel portion of a transistor cannot be or is hard to be accurately formed in a crystal grain, and hence there is a problem that characteristics of the transistor are deteriorated.
Moreover, when the phase shifter having a phase difference of 180 degrees is used and when the phase shifter having a phase difference of 60 degrees is used, an unnecessary protruding peak shape is generated on both sides of the inverse peak in the light intensity distribution with the inverse peak shape in, e.g., the focus state as indicated by circles of broken lines in FIGS. 44C and 45C. That is, this unnecessary peak shape corresponds to a high light intensity part. When such a peak shape exists on both sides or one side of the inverse peak in the light intensity distribution with the inverse peak shape, since the light intensity becomes large at this peak shape part only, ablation occurs and the semiconductor film is disadvantageously broken. Additionally, when a crystallized semiconductor film is generated by applying a light intensity distribution with an inverse peak shape to a non-crystal semiconductor film, since crystal growth which has started in the lateral direction from a minimum intensity area at an inverse peak part stops at a descending gradient part of a peak shape part with a high intensity, there is a disadvantage that a crystal with a large particle size cannot be generated.